tick management: spread timer interrupt

After discussing w/ Thomas over IRC, it seems the issue is the sched tick
fires on every cpu at the same time, causing extra lock contention.

This smaller change, adds an extra offset per cpu so the ticks don't line up.
This patch also drops the idle latency from 40us down to under 20us.

Signed-off-by: john stultz <johnstul@us.ibm.com>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Cc: Ingo Molnar <mingo@elte.hu>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

kernel/time/tick-sched.c

@@ -546,6 +546,7 @@ void tick_setup_sched_timer(void)
 {
 	struct tick_sched *ts = &__get_cpu_var(tick_cpu_sched);
 	ktime_t now = ktime_get();
+	u64 offset;
 
 	/*
 	 * Emulate tick processing via per-CPU hrtimers:
@@ -554,8 +555,12 @@ void tick_setup_sched_timer(void)
 	ts->sched_timer.function = tick_sched_timer;
 	ts->sched_timer.cb_mode = HRTIMER_CB_IRQSAFE_NO_SOFTIRQ;
 
-	/* Get the next period */
+	/* Get the next period (per cpu) */
 	ts->sched_timer.expires = tick_init_jiffy_update();
+	offset = ktime_to_ns(tick_period) >> 1;
+	do_div(offset, NR_CPUS);
+	offset *= smp_processor_id();
+	ts->sched_timer.expires = ktime_add_ns(ts->sched_timer.expires, offset);
 
 	for (;;) {
 		hrtimer_forward(&ts->sched_timer, now, tick_period);